PROJECT SUMMARY The long-term goal of the proposed work is to gain a better understanding of how gene expression is regulated during mammalian cell development. This proposal focuses on the study of post-transcriptional control of gene expression, whereby mRNAs made from a single gene can be modified in different ways to change gene output. In the nucleus, this includes altering the protein-coding and non-coding sequences of mRNA. In the cytoplasm, this includes regulatory events that dictate which mRNAs are preferentially degraded or translated to make proteins. Together, these regulatory processes specify the identity and abundance of proteins present in each cell, and consequently, cell properties. In order for cells to progress through different stages of development, its mRNAs undergo extensive stage-specific regulation. Defects in mRNA regulation are the direct cause of many human diseases, thus an understanding of how mRNAs are regulated is essential. In the proposed study, we will comprehensively characterize mRNA regulatory events that drive cells through different stages of male germ cell development. This cellular program depends on undefined mRNA regulatory programs. Our plan is to isolate postnatal mouse germ cells at different steps in this pathway and use transcriptome-wide tools to 1) identify changes in mRNA expression and translation, 2) understand the functional significance of these changes, and 3) determine how these changes are controlled at the molecular level. We will use an approach established in our laboratory that combines dual fluorescence cell labeling in transgenic mice and fluorescence activated cell sorting to isolate cells at different stages of development. Combining this approach with deep sequencing, biochemical, and bioinformatic tools, we will reveal multiple layers of mRNA-based gene control during germ cell development. Altogether, we will gain important insights into the functions and mechanisms of mRNA regulation in mammalian cell development in unprecedented molecular and cellular detail. As a result, this study will lead to an improved understanding of how gene expression is controlled through mRNA regulation during germ cell development. The data has the potential to provide important new insights into human reproductive biology and fertility. This study will also reveal molecular mechanisms controlling mammalian cell differentiation and proliferation, and therefore will be of direct relevance to human development and disease.